Over the preceding 60 years, numerous large earthquakes have caused collapses and loss of life in structures that were designed and built in accordance with the custom and practice of the industry at the time of their construction. Investigation and analysis of these failures identified several critical deficiencies in these customs and practices:
1. Multi-story buildings with weak, open, or soft stories.
2. Buildings with irregular shapes, distribution of mass, location of lateral load resisting systems, et cetera.
3. Inadequate resistance to lateral loads
4. Inadequate connections between structural members and lateral load resisting systems.
The cost of the damage and loss of life in these buildings caused by a basic service earthquake (BSE) far exceeds the ability of individuals, building owners, and insurance companies to compensate. Consequently, the cost of such compensation has been borne by local, state, and federal governments.
In response to these developments, local and state governments have passed laws that require or recommend the retrofit of such buildings to prevent collapse in a BSE-2 (2% probability of exceedance in 50 years) and loss of life in BSE-1 (10% probability of exceedance in 50 years). Insurance companies will not insure such buildings until they are retrofitted. And banks might not accept such buildings as collateral for loans until they are retrofitted.
The conventional methods for seismically retrofitting such buildings include the following methods:
1. Fully rebuilding the existing structure to meet current requirements.
2. Partially rebuilding the existing structure to prevent collapse yet allow such damage as to render the building valueless.
3. Installing special moment resisting frames & foundations in parallel with the existing structure to resist the seismic loads.
4. Installing conventional concentrically braced frames & foundations in parallel with the existing structure to resist the seismic loads.
5. Installing tuned masses and/or dampers to reduce the effect of seismic loads.
6. Installing base isolators to separate the building from the seismic loads.
7. Installing eccentrically braced frames & foundations in parallel with the existing structure to resist the seismic loads.
The inherent problem with such design and retrofit approaches is the lack of force distribution control that is needed in buildings with soft, weak, open, or irregular stories to adequately tune the stiffness of the superstructure with added lateral resisting elements such as those listed above. In other words, there are no controlled interactions between the two systems (existing and newly added) to avoid local failures and/or fine tune the required harmonic and tolerated displacements of the entire building as a single system. In theory, if one could retrofit the soft, weak, open, or irregular story of the superstructure proportionally along the height of the building, the damage and loss of life in these buildings caused by a BSE due to premature collapse of the soft, weak, open, or irregular story could be avoided.
Additionally, the total cost of the aforementioned conventional retrofits typically outweighs their short-term economic benefit or their amortized cost exceeds the available income generated by the building. The result is laws, rules, and regulations that are not properly enforced, that are delayed, or that are retracted (or not passed in the first place) in the face of opposition from building owners who claim economic hardship, inverse condemnation, or unconstitutional taking. When insurance and financing are denied, such buildings become public nuisances.
Accordingly, there is a need in the art for a low cost system that can be used to retrofit buildings and increase their earthquake resistance.